I use a lot of latex for props. As far as foam goes, I use those big 4x8' pink sheets at home depot for a lot of stuff. I still haven't figured out where to get big foam blocks for carving stuff out. I know some people buy the stuff for couch cushions, but I want to get something more for large theater props.

Have you seen my youtube channel? I cover alot of materials and techniques and am very budget minded.
Plastic, tinfoil, caulk can all make great props for a haunt. Not to mention traditional things like paper mache.
here is a link to my channel, I have tried to name everything so it is self explanatory. Check out ROUS, Ogre head, Halloween scarecrow, and my Kappa videos for full size cheapo props.
If you mention a specific thing you would like to build I would be happy to help you.http://www.youtube.com/user/Stiltbea...s?feature=mhee